Conventional Electric compressors, such as HVAC compressors for vehicles are typically associated with an electric motor.
An example of such electric motor driven compressor is given in patent document U.S. Pat. No. 6,183,215 B1.
Such types of electric motor driven compressors have many drawbacks linked to lubrication, refrigerants, low operating speed, friction losses and loss of compactness.
Two main categories of HVAC electric compressors are used in HEV/EV vehicles: rotary like vane compressors and oscillating like scroll type compressors.
Both types of electric compressors which are associated with an electric motor have the following drawbacks:                Lubricants are needed for different mechanical parts,        It is necessary to check compatibility of lubricant oil with refrigerants (such as for example the haloalkane refrigerant R134a or the more recently used hydrofluoroolefin refrigerant HFO-1234yf),        Lubricants should be carefully chosen to protect the electric motor windings from the risk of insulation failure,        An oil separator and leak detection devices are required to avoid contamination of the electric systems in EV/HEV vehicles,        The conventional electric compressors have a speed which is limited and cannot exceed 10,000 rpm,        Friction losses are detrimental to the operation of the electric compressor.        
Furthermore patent document US 2002/0040581 A1 discloses an exhaust gas turbocharger for an internal combustion engine having an exhaust gas turbine arranged in an exhaust gas line, and a compressor arranged in an intake duct and connected to the turbine by way of a shaft, the compressor including a compressor wheel forming at the same time a rotor of an electric motor.
Patent document EP 1201891 A1 further discloses a turbocharger with electrical assistance which uses a wheel of the turbocharger as induction motor rotor with windings to create a magnetic field.